What it takes to find crystals almost as old as Earth itself
In 1981, professor Ajit Kumar Saha and his associates (colleagues and senior research students from Presidency College, Calcutta) published a research paper in the journal Science, claiming to have found rocks in Champua, Orissa, that were about 3,800 million (3.8 billion) years old.
The global geological community was sceptical. The rocks were probably old, but could they be that old? Experts from across India and around the world began to head to Orissa. In 2000, Rajat Mazumder, then a lecturer in Kolkata, started focusing on these rocks. In 2010, he was joined by Trisrota Chaudhuri, 35, then with the Indian Statistical Institute.
“We were confident that the rocks of the Champua region were even older than previously thought,” says Mazumder, 53, now an associate professor of applied geosciences at the German University of Technology in Oman.
They were right. In 2018, they published a paper in the journal Scientific Reports on two zircon crystals they extracted from rocks taken from the Champua site. While the rocks were 3.4 billion years old, the crystals were much older, at an estimated 4 and 4.2 billion years old.
It has since been established that these zircons, along with those reported from the Acasta Gneiss in Canada and the Jack Hills in Australia (both about 4.4 billion years old) are the oldest recorded minerals in known geological history. Excerpts from an interview with Mazumder and Chaudhuri.
What do the zircons you found in Champua tell us about early earth?
Chaudhuri: The first 500 million years of earth’s history, the Hadean Eon, was truly “hellish”. Asteroids constantly bombarded the earth and the surface was covered by an ocean of lava. Temperatures were so high (over 2,000 degrees C) that rocks vapourised.
The Hadean is considered a “dark age” in geological history because we know very little about this time.
It was earlier believed that the surface was nearly homogeneous at this time. Studies like ours suggest that, despite the extreme conditions, some parts of the Earth’s surface started producing rocks somewhat similar to modern-day rocks, which survived. It also suggests that the production of modern-day-like rocks started much earlier than previously estimated.
Why are zircons so significant in geology?
Mazumder: Zircon is a mix of silicon, oxygen and a metal called zirconium. It is a mineral formed by the crystallisation of magma from deep within the earth’s crust. Each crystal, generally less than 1 mm in size, is highly resistant and acts as a snapshot of geological events that occurred in the distant past. Sometimes, younger rocks can contain zircon derived from older rocks, which have since eroded away.
Chaudhuri: Zircons outlast the rocks that hosted them. It’s like a ripe fruit fallen on the ground. Eventually the soft flesh rots away but the seed remains, proving that the fruit was once there. No rock more than 4,000 million years old survives today. All the “oldest rocks” have only left behind traces of zircon, an extraordinarily robust mineral and can survive hell [high pressures and temperatures].
What does it take to accurately date rocks this old?
Chaudhuri: The oldest rocks are more likely to occur in ‘pockets’ within younger rocks. We separate zircons via several treatments. Then the zircons are probed in an instrument called the SHRIMP (Sensitive High Resolution Ion Microprobe). In our case, this stage was done by professor Yusheng Wan and his colleagues at the Beijing SHRIMP Centre. We pursued several renowned laboratories around the world before we reached them, but we could not convince the other labs that these rocks were worth looking into.
What has happened to the site of these discoveries?
Mazumder: Nothing has happened. I am not aware of any move to preserve it. After our discovery, I contacted several government bodies and research institutes in India to preserve and sponsor further research, but have received no response.
I have managed to secure funding from the Research Council of Oman to work on these rocks, but am currently looking for a collaborator from India to help carry out intensive studies.
The Champua zircons and geology of Singhbhum are of global importance. Last February, before the lockdown, researchers from the German Research Foundation’s Building a Habitable Earth project visited many parts of Jharkhand and Odisha. I led part of a field workshop. Some of the participants, experts on early-earth research, were surprised to know that such sites were not preserved, and have shown interest in collaboration, which has been on the backfoot though because of the pandemic.